System and method for transferring data on a data link

ABSTRACT

A system and method are provided for transferring a packet across a data link. The packet may include a stream of data symbols which is delimited by one or more framing symbols. Corruptions of the framing symbol which result in valid data symbols may be mapped to invalid symbols. If it is desired to transfer one of the valid data symbols that has been mapped to an invalid symbol, the data symbol may be replaced with an unused symbol. At the receiving end, these unused symbols are replaced with the corresponding valid data symbols. The data stream of the packet may be encoded with forward error correction information to detect and correct errors in the data stream.

RELATED APPLICATIONS

[0001] This application claims the benefit under Title 35 U.S.C. §119(e) of U.S. Provisional Application Serial No. 60/391,340, filed Jun.25, 2002, entitled “System and Method for Transferring Data on a DataLink” by Robert M. Cole and James E. Bishop, the contents of which arehereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

[0002] The Government of the United States of America has rights in thisinvention pursuant to contract no. NAS9-20000, awarded by the NationalAeronautics and Space Administration (NASA).

FIELD OF THE INVENTION

[0003] The present invention relates to computer networks and, moreparticularly, to high speed data links for connecting nodes in acomputer network.

BACKGROUND OF THE INVENTION

[0004] With the widespread use of computer networks, it is oftennecessary to transfer large quantities of data between computer systemsat high speeds. Many networking specifications have been developed toprovide high data transfer rates. Typically, such specifications includehardware and software specifications. The hardware specificationtypically describes the network at the physical level, includinginformation about the physical link and its electrical characteristics,types of connectors that may be used, and electrical parameters forvarious data rates. The software specification usually describes anetwork protocol through which nodes in the network may communicate andmay include information about frame formats, sequence management, flowcontrol, and topologies. One such networking specification is Ethernet.Ethernet is widely used in Local Area Networks (LANs), but often cannotsupport data rates capable of meeting the demands of some computernetworks, particularly those which require real-time data transfer.

[0005] Another networking specification is Fibre Channel. Although thelower layers of the Fibre Channel protocol allow for high data rates,the upper layers of the Fibre Channel protocol require talk back betweena receiving node and a sending node for control purposes, such asacknowledging receipt of data, notification of bad packets, and deliveryof “busy” messages when a receiver is not ready to receive data. Thisadditional overhead increases network traffic and transfer times.Moreover, Fibre Channel specifies a maximum bit error rate (BER) of1×10⁻¹². Such a BER may not be suitable for some applications whichrequire orders of magnitude higher reliability.

SUMMARY OF THE INVENTION

[0006] According to one aspect of the invention, a method is providedfor transferring on a data link a packet having a plurality of datasymbols from a set of symbols and at least one framing symbol from theset of symbols, the set of symbols including a subset of valid symbolsand a subset of invalid symbols, the method comprising mapping at leastsome possible corruptions of the at least one framing symbol that resultin valid symbols from the subset of valid symbols to the subset ofinvalid symbols, and replacing, in the packet, at least some of thevalid symbols mapped in to the subset of invalid symbols with unusedsymbols in the set of symbols.

[0007] According to another aspect of the invention, the method furthercomprises adding error correction information pertaining to theplurality of data symbols to the packet. According to another aspect ofthe invention, the method further comprises encoding the packet in aformat suitable for transmission. According to another aspect of theinvention, the method further comprises transferring the packet on theserial link. According to another aspect of the invention, the methodfurther comprises indicating an error in the packet if the packetincludes a symbol from the subset of invalid symbols. According toanother aspect of the invention, the method further comprisesdetermining if the error in the packet is in a data symbol or framingsymbol based on the location of the error in the packet. According toanother aspect of the invention, the method further comprises recoveringthe valid symbols mapped into the subset of invalid symbols by replacingat least some of the unused symbols in the packet with the valid symbolsmapped into the subset of invalid symbols.

[0008] According to one aspect of the invention, a system is providedfor transferring on a data link a packet having a plurality of datasymbols from a set of symbols and at least one framing symbol from theset of symbols, the set of symbols including a subset of valid symbolsand a subset of invalid symbols, the system comprising a mapper formapping at least some possible corruptions of the at least one framingsymbol that result in valid symbols from the subset of valid symbols tothe subset of invalid symbols, and a first character translation unitfor replacing, in the packet, at least some of the valid symbols mappedin to the subset of invalid symbols with unused symbols in the set ofsymbols. According to another aspect of the invention, the systemfurther comprises a first forward error correction unit for adding errorcorrection information pertaining to the plurality of data symbols tothe packet. According to another aspect of the invention, the systemfurther comprises an encoder for encoding the packet in a formatsuitable for transmission. According to another aspect of the invention,the system further comprises a transmitter for sending the packet on theserial link, and a receiver for receiving the packet from the seriallink. According to another aspect of the invention, the system furthercomprises an error detection unit for indicating an error in the packetif the packet includes a symbol from the subset of invalid symbols.

[0009] According to one aspect of the invention, a system is providedwherein the error detection unit further comprises an error resolutionunit for determining if the error in the packet is in a data symbol orframing symbol based on the location of the error in the packet.According to another aspect of the invention, the system furthercomprises a second character translation unit for recovering the validsymbols mapped into the subset of invalid symbols by replacing at leastsome of the unused symbols in the packet with the valid symbols mappedinto the subset of invalid symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram of a data link using 8 b/ 10 b encodingaccording to one embodiment of the invention;

[0011]FIG. 2 is a block diagram of a computer system for transferringdata over a data link according to one embodiment of the invention;

[0012]FIG. 3 is a block diagram of a computer system using aunidirectional serial data according to one embodiment of the invention;

[0013]FIG. 4 if a block diagram of a computer system for transferringdata over a data link according to one embodiment of the invention;

[0014]FIG. 5 is a diagram of an example of a transaction packetaccording to one embodiment of the invention;

[0015] FIGS. 6A-6D are diagrams of transaction packets having invalidsymbols according to one embodiment of the invention;

[0016]FIG. 7 is a block diagram of computer system using forward errorcorrection and character translation for data transfer according to oneembodiment of the invention;

[0017]FIG. 8 is a flowchart illustrating an example of a method fortransferring data using the computer system of FIG. 7;

[0018]FIG. 9 is a block diagram illustrating an example of addingforward error correction information to a transaction packet, accordingto one embodiment of the invention; and

[0019]FIG. 10 is a block diagram illustrating an example of forwarderror correction with multiple FEC decoders, according to one embodimentof the invention.

DETAILED DESCRIPTION

[0020] Digital communications transmitted over communications media, forexample, coaxial cabling, twisted pair cabling, or fiber optic media,are often encoded to facilitate clock recovery, error detection, andmaintain DC balance in the cable. Encoding involves the conversion ofthe bit stream of the data signal to symbols having signalcharacteristics which facilitate reception. Such characteristics mayinclude a limited maximum run length and a limited DC imbalance.

[0021] DC imbalance can be described as the ratio of logical ones tological zeros in a particular symbol. The maximum run length is themaximum number of consecutive logical ones or logical zeros in a symbol.Controlling DC imbalance and maximum run length can be important becausewithout DC balance, inter-symbol interference may result from asingle-bit transition and cause a bit error. The error occurs because asingle-bit transition, after a long string of no transitions, may notcontain the energy necessary to change the stored charge through theentire cable.

[0022] Also, controlling run length and DC balance may be important forsynchronization purposes. A receiver can lose synchronization with atransmitter if too many intervals elapse without a bit transition. Thisloss of synchronization generally happens because clock recovery at thereceiver relies upon transitions between logical one and logical zero todetect the clock frequency.

[0023] Nb/Mb encoding involves the encoding of each N-bit block of datainto an M-bit symbol. For example, in 8 b/ 10 b encoding, 8-bit datablocks are encoded as 10-bit symbols at the transmitting end and the10-bit symbols are transferred on the data link. At the receiving end,the 10-bit symbols are received and decoded into the original 8-bit datablocks. FIG. 1 illustrates a simple point-to-point network connection inwhich network device 102 and network device 110 are connected throughdata link 106. Network device 102 and network device 110 processinformation only in the 8-bit domain. Data is encoded into 10-bitsymbols by 8 b/ 10 b encoder 104 and sent across the data link. Prior toreception of the data by network device 110, the 10-bit symbols aredecoded by 10 b/ 8 b decoder 108. Encoder 104 and decoder 108 may, forexample, be implemented as part of the transceivers which send the data.Thus, all data on the data link is in the 10-bit domain. The data linkdepicted in FIG. 1 is unidirectional. However, it should be understoodthat a bidirectional data link could also be used, wherein encoder 104could also provide 10 b/ 8 b decoding and decoder 108 could additionallyprovide 8 b/ 10 b encoding. Moreover, FIG. 1 depicts a simplepoint-to-point network topology with two network devices. It should beunderstood that any number of network devices connected in any networktopology, such as a star topology, ring topology, bus topology, or treetopology, could be used. Moreover, any type of network device, such as,for example, workstations, servers, routers, or switches could be used.Additionally, other Nb/Mb encoding schemes could be used. For examples,7 b/ 10 b encoding could be used or 4 b/ 5 b encoding could be used.

[0024] Typically, in Nb/Mb encoding, not every M-bit symbol is be used.Some of the unused symbols may be designated as invalid symbols. Thus,transmission of an invalid symbol can be used to detect errors, such ascorruption of valid symbols by noise. For example, in an 8 b/ 10 bencoding scheme, there are 256 possible different 8-bit data blocks.However, there are 1024 possible 10-bit symbols. For each possible 8-bitdata block, there may be a positive disparity 10-bit symbol and anegative disparity 10-bit symbol which represent that block to helpmaintain DC balance on the transmission media. That is, if the runningdisparity on the cable is +1, then the transmitter may send a negativedisparity version of the next symbol to bring the running disparity backdown to 0. Even with two 10-bit symbols for each 8-bit data block, only512 symbols out of 1024 possible symbols are used. However, thetransmission protocol may require some control symbols which can be usedfor data framing or other control purposes. Still, it is likely thatthere will be some 10-bit symbols which are not used as either datasymbols or control symbols. These symbols can be used as symbols thatare invalid for transmission. Symbols in this set of invalid symbols arenever intentionally transmitted. If any invalid symbol is received atthe receiving end, it can be determined that a valid symbol (e.g., adata or control symbol) has been corrupted.

[0025] Although some errors may be detected by the receipt of invalidsymbols, corruptions of valid symbols may sometimes result in othervalid symbols. For example, in a system which uses 4-bit data symbols,suppose 0110 and 0111 are both valid data symbols and 1110 is an invalidsymbol. Further, suppose the first bit of the symbol 0110 is corruptedfrom a logical ‘0’ to a logical ‘1’ during transmission. The resultingsymbol, 1110, is an invalid symbol and will be detected as such at thereceiving end. If, however, the last bit of the symbol 0110 is corruptedfrom a logical ‘0’ to a logical ‘1’ during transmission the result is avalid symbol (i.e., 0111) and will be perceived as such by the receiver,despite the fact that it is a corrupted symbol. As a result, it may alsobe necessary to provide a way to detect corruptions of valid symbol,which result in valid symbols. One way in which such errors can bedetected is by monitoring the running disparity on the cable. By havingpositive-disparity and negative-disparity versions of each data block,DC balance on the cable can be maintained. As a result, if the runningdisparity of the cable exceeds a particular threshold, then it can bedetermined that an error occurred in transmission. Although receipt ofinvalid symbols and excessive running disparity allows for some errordetection, it may be difficult or impossible to determine which symbolwas intended for transmission and which bits in the symbol werecorrupted. Further, the running disparity exceeding the predeterminedthreshold as a result of a corrupted symbol may not occur for a longperiod of time after the corrupted symbol was received.

[0026] To help correct such errors, a forward error correction (FEC)algorithm may be used. Forward error correction algorithms typicallyencode blocks of data so that any corruption of the block of data duringtransmission can be resolved at the receiving end. Examples of FECalgorithms which may be used in accordance with various embodiments ofthe invention include block code algorithms, cyclic code algorithms,Bose-Chaudhuri-Hocquenghem (BCH) algorithms, Viterbi coding algorithms,and convolutional code algorithms. Although applying FEC algorithms to ablock of data may increase the size of the block of data (e.g., byadding error correction information), such algorithms provide a way todetect and correct errors upon reception.

[0027] An FEC algorithm may be applied to the data before Nb/Mbencoding, thus allowing valid M-bit symbols to be transmitted on thedata link. At the receiving end, the symbols may first be decoded intoN-bit blocks before using the FEC algorithm to detect and correct errorsand recover the original N-bit blocks.

[0028] As mentioned above, many transmission protocols provide specialcontrol symbols which can be used for data framing or other controlpurposes. If such control symbols are used to frame the N-bit blocks ofdata, it may not be possible to apply an FEC algorithm to the controlsymbols because the FEC encoder requires use of the control symbols todefine the data block to encode. As a result, the control symbols arenot encoded with any forward error correction information. However, insome situations, it may be necessary to detect corruptions of thecontrol symbols used for framing that occurred during transmission toachieve a desired BER. To provide error detection ability for thesecontrol symbols, possible corruptions of the control symbol used forframing that result in valid M-bit symbols may be mapped to the set ofinvalid symbols.

[0029] For example, some corruptions of the control symbol used forframing, which may be referred to herein as the framing symbol, mayresult in invalid M-bit symbols, while others result in valid datasymbols or other valid control symbols. If corruption of the framingsymbol results in an invalid symbol, then reception of invalid symbol atthe receiving end will result in detection of an error. However, ifcorruption of the framing symbol results in a valid symbol, it is stilldesirable to detect a corruption of the framing symbol. Thus, validsymbols in which a corruption of the framing symbol may result can bemapped into the set of invalid symbols. As a result, when a framingsymbol is corrupted into one of the valid symbols that has been mappedinto the set of invalid symbols, the receiver can detect an error.However, an intentional transmission of one of the mapped valid symbolscould also result in a detection of an error when one has not occurred.To prevent such situations, valid symbols which have been mapped intothe set of invalid symbols may not be transmitted intentionally. If itis desired to transmit data which requires use of one of these symbols,a character translation unit may replace the symbol with an unusedcontrol symbol or an unused data symbol. Character translation will bediscussed below in greater detail.

[0030]FIG. 2 illustrates an example of a transmission using a charactertranslation unit. FEC encoder 200 receives data from the transmittingagent and applies an error correction algorithm to blocks of datadefined by the framing symbol and passes the FEC encoded data tocharacter translation unit 202. Character translation unit 202determines if the FEC encoded data includes any symbols which have beenmapped into the set of invalid symbols. If so, then these symbols arereplaced with corresponding unused symbols and the data is passed totransmitter 204. If no mapped valid symbols are included in the datablock then character translation unit 202 simply passes the data totransmitter 204. Transmitter 204 performs Nb/b encoding on the data andtransmits the data on data link 212. At the receiving end, receiver 206performs Mb/Nb decoding on the data and passes the data to charactertranslation unit 208. Character translation unit 208 performs anopposite operation to that of character translation unit 202. That is,any unused symbols that were used to replace mapped valid symbols arereplaced with the corresponding mapped valid symbols. Then, the data ispassed to FEC decoder 210 which applies the FEC algorithm to decode thedata and detect and correct errors in the data symbols. If all errorsare corrected, the output of FEC decoder 210 is the original data thatwas passed to FEC encoder 200.

[0031] One embodiment of the invention will now be described withrespect to a data link in a space shuttle. However, many otherembodiments will readily occur to one skilled in the art and areintended to be within the spirit and scope of the invention. Forexample, embodiments of the invention could be used in situations inwhich a highly reliable and high speed data link is required. Suchsituations could include transfer of banking information, such aselectronic checking or electronic transfer of funds information. Otherapplications in which the present invention could be used includecomputer networks, such as local area networks (LANs), Wide AreaNetworks (WANs) and Storage Area Networks (SANs). Moreover, in variousembodiments of the invention, it is not required that a physical cablebe used to transfer data. For example, various embodiments of theinvention may be used with air as the transfer medium, such as inwireless networks. It should be appreciated that the invention is notlimited to any particular application and various embodiments of theinvention are suitable for use in any application in which it isdesirable to transfer data in a reliable manner.

[0032] As the complexity of space shuttle missions increases, theprocessing demands on space shuttle general purpose computers (GPCs)have also increased. Because of the high reliability of the GPCs it isnot desirable to replace or significantly modify them, but it is stillnecessary to increase their processing power. To increase the processingpower of the GPCs, a command and display processor (CDP) may be added.The CDP is a second processing which offloads some of the processingdemands of the GPCs. For example, the CDP may handle caution and warningprocessing and safety of flight processing. The CDP can receiveinformation from an avionics ground equipment (AGE) port on the GPCs andcan copy all memory transactions to its own memory to create andmaintain an exact copy of the GPCs memory. An avionics ground equipmentsupport (AGES) unit may interface with the AGE port on the GPC,serialize data received from the GPC and provide the information. TheAGE port on the GPC is a unidirectional “listen only” test port. Becausethe AGE port is unidirectional, it is desirable that the data linkbetween the GPC and CDP run in real-time and not rely on anyre-transmission requests. Also, because weight restrictions exist on theshuttle, it is desirable that the data link be lightweight.

[0033]FIG. 3 is a block diagram of a data link between the GPC and CDPaccording to one embodiment of the invention. In one embodiment of theinvention, a serial data link 306 may be used to reduce weight. In aserial data link using differential signaling, two wires may be used.However, it is also possible to use a parallel data link having morethan two wires. Moreover, although the present example is described inthe context of unidirectional transmission, the invention is not limitedin this respect. For example, various embodiments of the invention maybe used with bi-directional data links. AGES unit 304 monitors the AGEtest port of GPC 302 and serializes the data for transmission overserial data link 306. Data is transmitted on data link 306 to CDP 308.

[0034] Because of the desire for high speed data transfer rates, theFibre Channel physical layer may be used to implement the serial datalink. However, because the upper layers of the Fibre Channel employ datare-transmission in case of lost data and because the AGE port is a“listen only” port, it is not desirable to use the upper layers of theFibre Channel transmission protocol. The Fibre Channel physical layer,FC-0, FC-1, and FC-2, specify the physical and electricalcharacteristics of the data link, the serial encoding and decoding ofdata, and basic signaling and data framing functions. Although thepresent example uses the physical layer of the Fibre Channelspecification, many other network specifications could be used in otherembodiments. For example, Ethernet or Gigabit Ethernet networkspecifications could be used.

[0035] As shown in FIG. 4, AGES logic 402 passes serialized data toFibre Channel Transmitter 404. Fibre Channel transmitter performs 8 b/10 b encoding on the data using 8 b/ 10 b encoder 406 and transmits thedata on data link 408. Fibre Channel receiver 410 receives data fromdata link, decodes the data with 10 b/ 8 b decoder 412, and passes thedata to CDP 414, which may then deserialize the data.

[0036] Fibre Channel encodes data using 8 b/ 10 b encoding scheme. Thatis, 8-bit blocks of data are encoded as 10-bit symbols for transmission.However, because 1024 symbols are available (using all combinations of10 bits) in the 10-bit domain, 512 symbols may be used such that eachsymbol has a positive disparity representation and a negative disparityrepresentation to maintain DC balance on the data link and facilitateclock recovery at the receiving end. Of the 512 symbols, 256 are datasymbols, 12 are control symbols, and 244 are invalid symbols. All validsymbols (e.g., the data symbols and control symbols) use either a 4/6,5/5, or 6/4 ratio of ones to zeros. Invalid symbols are neverintentionally transmitted. One control symbol, K28.5, is used for clocksynchronization between the transmitter and receiver.

[0037] Although Fibre Channel specifics 8 b/ 10 b encoding, a 7 b/ 10 bencoding scheme could also be used. Because there are 138 5/5 10-bitsymbols and only 128 possible 7-bit data characters, it is possible touse a 7 b/ 10 b encoding scheme where each 7-bit data charactercorresponds to a 5/5 10-bit symbol. In such a scheme, a 7-bit datacharacter is first encoded as an 8-bit data character. Then, the usual 8b/ 10 b encoding may occur. At the receiving end, a 10-bit symbol isdecoded into its corresponding 8-bit data character. The 8-bit datacharacter may then be decoded to the original 7-bit character. Also, ten5/5 10-bit symbols may be used for control characters. If only the 5/5data symbols and 5/5 control symbols are transferred on the data link,any single-bit corruption of any data symbol will result in either a 6/4symbol or a 4/6 symbol. As a result, if a 6/4 symbol or a 4/6 symbol isreceived, an error can be detected. Likewise, if any symbol other than a5/5 symbol is received, such as a 7/3 symbol or an 8/2 symbol, then anerror can be detected in these symbols upon reception.

[0038] According to one embodiment of the invention, the maximum datarate of the data link may be one transaction every 250 nanoseconds. Thespeed of the serial link may be one symbol every 18.8 nanoseconds. As aresult, a maximum of 13 symbols can be transmitted within the 250nanosecond transaction time. FIG. 5 is a diagram which illustrates anexample of a 13-symbol transaction packet 502. Transaction packet 502includes three control symbols, 504, 508, and 510. The control symbolsmay be K28.5 framing symbols used for data framing and facilitation ofclock synchronization. Transaction packet 502 may also include ten datasymbols, 506 a-506 j. Of the ten data symbols, two may be FEC redundancysymbols to allow for correction of any corruption of a single corrupteddata symbol. The FEC redundancy symbols may be, for example, the lasttwo data symbols in the transaction packet (e.g., 506 h and 506 j) ormay be located elsewhere in the transaction packet.

[0039] One forward error correction algorithm that may be used is aReed-Solomon-based algorithm. A Reed-Solomon encoder can take as inputdata streams which include k symbols of m bits each and output a datastream which includes n symbols of m bits each. Thus, n-k symbols, whichinclude redundancy information, may be added to the original data streamby the Reed-Solomon decoder. Based on this data stream, the Reed-Solomondecoder can correct (n-k)/2 symbols in the received data stream. In oneembodiment of the invention, a Reed-Solomon encoder is used to encodedata streams to create transaction packets of thirteen symbols. Asmentioned above, before the transaction packet is 8 b/ 10 b encoded, itincludes ten eight-bit data symbols and three eight-bit K28.5 framingsymbols. There are 256 possible eight-bit characters. However, the FibreChannel specification requires 256 data characters and 12 controlcharacters. To represent each of these 256 data characters and 12control characters in the eight-bit domain using only 256 eight-bitcharacters, eight-bit symbols may be used. An eight-bit symbol includesan eight-bit character plus an additional 3-bit qualifier whichindicates whether the eight-bit character is a data character or acontrol character.

[0040]FIG. 9 is a block diagram illustrating an example of addingforward error correction information to a transaction packet, accordingto one embodiment of the invention. A transaction packet 902 includes adata stream made up of data symbols. Transaction packet 902, whichinitially includes eight eight-bit data symbols, is passed to theReed-Solomon encoder 904. Reed-Solomon encoder 904 adds two eight-bitdata symbols 905 a and 905 b to the transaction packet, resulting intransaction packet 906. These two symbols, 905 a and 905 b, may includeredundancy information which is used at the receiving end to detect andcorrect errors that occurred during transmission. As a result,transaction packet 906 will include the eight original data symbols, andtwo FEC symbols. Thus, as mentioned above, n-k/2 symbols, or any onesymbol, in transaction packet 906, may be corrected. Transmitter 916transmits a stream of K28.5 control symbols 912. When transmitter 916receives a transaction packet, such as transaction packet 912, itimposes the transaction packet on the stream of K28.5 control symbols.As a result, the transaction packet is delimited by K28.5 controlsymbols. Transaction packet 914 is formed by transmitter 916 and istransmitted across the channel.

[0041] Framing control block 909 receives the transaction packet anddetermines whether the transaction packet includes any invalid symbols.If so, framing control block 909 determines if the invalid symbols aredata symbols or control symbols. If, as shown in FIG. 6A, the invalidsymbol 604 is received between two data symbols in the transactionpacket (i.e., if the preceding and following symbols to the invalidsymbol are data symbols), it may be inferred that the invalid symbol isa corrupted data symbol. The transaction packet may then be passed tothe Reed-Solomon decoder 908 which may detect and correct the invaliddata symbol. If, as shown in FIG. 6B, an invalid symbol 610 is receivedbetween two control symbols (e.g., 608 a and 608 b), it may be inferredthat the invalid symbol is a K28.5 symbol and, as a result, framing maybe preserved. If, as illustrated in FIG. 6C, an invalid symbol 614 isreceived at the start of the data stream, and the following nine symbolsare data symbols, and the tenth subsequent symbol is a K28.5 symbol,then the invalid symbol may inferred to be a data symbol. Thetransaction packet may then be passed to the Reed-Solomon decoder forerror detection and correction. If, however, the following ten symbolsare data symbols, then the invalid symbol may be inferred to be a K28.5symbol and framing is preserved. If, as shown in FIG. 6D, an invalidsymbol 620 is received at the end of the data stream, then the invalidcharacter is inferred to be a data symbol because the previous ninesymbols were data symbols. The transaction packet may then be passed tothe Reed-Solomon decoder which can detect and correct the bad symbol.

[0042] In one embodiment of the invention, the Reed-Solomonencoder/decoder may be implemented so that any single symbol corruptionin a transaction packet and some two-symbol corruptions in a transactionpacket may be corrected. As described above, by including twoReed-Solomon redundancy symbols in a 10-symbol data stream, any onesymbol in the data stream may be corrected. Each symbol includes aneight-bit data character. To correct errors in the data stream, eacheight-bit data character may be divided into two four-bit datacharacters. Thus, for each eight-bit data character, there is an upperfour-bit data character, including the upper four bits, and a lowerfour-bit data character, including the lower four bits. These four-bitdata characters may treated as the symbols for two four-bit wideReed-Solomon encoders/decoders operating in parallel. A first of thefour-bit wide Reed-Solomon encoders/decoders may operate on all of theupper four-bit symbols of the data stream, while the second Reed-Solomonencoder/decoder operates on all of the lower four-bit symbols. Becauseany corruption of a single eight-bit symbol can be corrected using asingle Reed-Solomon decoder, any corruption of any single four-bitsymbol can be corrected.

[0043] Further, it also possible to correct some errors in two differenteight-bit characters. For example, suppose that the upper four-bitsymbol of a first eight-bit data character of the data stream iscorrupted. This error can be corrected by the first Reed-Solomondecoder, as long as no other upper four-bit symbols in any of the othereight-bit data characters in the data stream are corrupted. Now, supposethat a lower four-bit symbol from a second eight-bit data character ofthe same data stream is corrupted. The second Reed-Solomon decoder whichis processing the lower four-bit symbols may correct this symbol as longas no other four-bit symbols in any of the other eight-bit datacharacters are corrupted. As a result, any single corrupted eight-bitdata character may be corrected and some corruptions of two eight-bitdata characters may be corrected, provided that the corruptions occur inopposite halves of the eight-bit data characters.

[0044]FIG. 10 is a block diagram illustrating an example of forwarderror correction with multiple FEC decoders according to one embodimentof the invention. Transaction packet 1002 includes a data stream havingeight eight-bit data symbols. FEC encoders 1004 a and 1004 b eachoperate on different portions of each data symbol in transaction packet1002. FEC encoders 1004 a and 1004 b add two error correction symbols togenerate transaction packet 1006. On the decoding end, a data symbol intransaction packet 1006, such as symbol 1007, may be divided in to anupper half 1009 a, and a lower half 1009 b. FEC decoder 1010 a canprocess and perform error correction on all upper halves of the datasymbols in transaction packet 1006. Likewise FEC decoder 1010 b canprocess and perform error correction on all the lower halves of the datasymbols in transaction packet 1006 using the FEC symbols. As a result,corrected transaction packet 1012 may be produced.

[0045] Although the above-described example uses two FECencoders/decoders operating on four-bit portions of an eight-bitcharacter many other variations are possible. For example, fourReed-Solomon encoders/decoders could be used, with each encoder/decoderprocessing a two-bit portion of an eight-bit data character. Likewise,eight Reed-Solomon encoders/decoders could be used, with eachencoder/decoder working on a one-bit portion of an eight-bit datacharacter. Moreover, it should be appreciated that the above-describedtechnique of using multiple FEC decoders on portions of a singlecharacter is not limited to eight-bit characters. Any size charactercould be used. Moreover, any number of FEC encoders/decoders could beused and the character to decoded may be split up into any number orsize portions. For example, 16-bit characters could be divided into twoeight-bit portions, using two FEC encoders/decoders. Similarly, 16-bitcharacters could be divided into four four-bit portions using four FECencoders/decoders. Moreover, although the above described example usesdata streams having eight data symbols and two FEC symbols, theinvention is not limited in this respect. Any suitable number of datasymbols may be included in the transaction packet. It should beappreciated that the number of FEC redundancy symbols may depend on thenumber of data symbols in the data stream and on the particular FECalgorithm being used. Furthermore, although the above-described exampleuses this technique in the context of Reed-Solomon forward errorcorrection, the invention is not limited in this respect, as variousembodiments of the invention could use the same technique with manyother different forward error correction algorithms, such as BCHalgorithms.

[0046] It should be appreciated that while the above described systemand method for forward error correction is used in a data link, theinvention is not limited in this respect. Indeed, various embodiments ofthe invention may be used in many other applications. For example,embodiments of the FEC technique described above may be used in manydigital signal processing applications, such as playback of compactdiscs or other digital audio, digital wireless communications, such ascellular phones or wireless computer networks.

[0047] Because there are two versions of the 10-bit K28.5 symbol, apositive disparity version and a negative disparity version, there are20 possible single-bit corruptions of the K28.5 symbol. It may not bepossible to use forward error correction, such as the Reed-Solomonalgorithm described above, on the K28.5 symbols because this symbol isused by the forward error correction decoder (e.g., the Reed-Solomondecoder) to define the data stream for decoding. Of these 20 possiblesingle-bit corruptions of the K28.5 eight result in an invalidtransmission symbol, eight result in a valid data symbol, and fourresult in a valid control symbol. The four single-bit corruptions of theK28.5 symbol that result in a control symbol result in two differentcontrol symbols. That is, one of these four single-bit corruptions ofthe K28.5 symbol results in the positive disparity version of a firstcontrol symbol, a second of these four single-bit corruptions of theK28.5 symbol results in the negative disparity version of the firstcontrol symbol. Likewise, a third of these four single-bit corruptionsresults in a positive disparity version of a second control symbol and afourth of these four single-bit corruptions results in the negativedisparity version of the second control symbol. These two controlsymbols are mapped into the set of invalid symbols. That is, they arenever intentionally transmitted and are treated as an invalid symbols atthe receiving end. As a result, of the original twelve control symbols,nine are left unused.

[0048] To prevent a corrupted K28.5 symbol from being interpreted as avalid symbol, the eight data symbols into which a K28.5 symbol can becorrupted are mapped into the set of invalid symbols. When it is desiredto intentionally transmit one of these eight data symbols, the datasymbols can be replaced with one of the unused control symbols. That is,each of the eight valid data symbols into which a K28.5 symbol can becorrupted may be assigned a corresponding control symbol from the set ofunused control symbols. When it is desired to transmit one of theseeight data symbols, it is replaced with its corresponding controlsymbol. As a result, any single-bit corruption of the K28.5 symbol willresult in an invalid symbol.

[0049] At the receiving end, if one of the nine unused control symbolsis received in a transaction packet, it may be replaced with itscorresponding data symbol. Then, Reed-Solomon decoding may be performedon the transaction packet to detect and correct any errors in datasymbols.

[0050]FIG. 7 is a block diagram illustrating an example of a computersystem which may be used in accordance with one embodiment of theinvention. FIG. 8 is a flow chart illustrating an example of a methodfor transfer of data using the system of FIG. 7. At step 802, anycorruptions of the framing symbol (e.g., K28.5 symbol) that result in avalid data symbol are mapped to the set of invalid symbols and unusedcontrol symbols are mapped to the valid data symbols that were mappedinto the set of invalid symbols. It should be understood that this stepneed not be performed every time data is to be transferred. Indeed, thisstep may be performed only one time. Once the mapping is established,modules such as character translation unit 712, receiver 710, or CDP 716(FIG. 7) can be programmed to acknowledge these mappings. Thus, anyfuture use of these modules may take the mappings into account. At step804, data is received from the transmitting agent. The transmittingagent may be, for example, AGES 702. At step 806, forward errorcorrection information is added to the data stream. Forward errorcorrection information may be added, for example, by Reed-Solomonencoder 704.

[0051] Next, at step 808 character translation is performed. Charactertranslation involves replacing any valid data symbols that were mappedinto the set of invalid symbols with corresponding unused symbols, suchas control symbols. Reed-Solomon encoder 704 and character translationunit 708 may be implemented, for example, in an application specificintegrated circuit (ASIC) or a programmable logic device (PLD), such asa field programmable gate array (FPGA). Moreover, Reed-Solomon encoder704 and character translation unit 708 may be implemented on the sameASIC or PLD or on separate ASICs or PLDs. Further, Reed-Solomon encoder704 and character translation unit 708 may be implemented as part ofAGES unit 702 or may be implemented as separate units. Charactertranslation unit 708 may output a transaction packet which, at step 810,is encoded using, for example, 8 b/ 10 b encoding, and transmitted overdata link 707. Transmitter 706 may be, for example, a commerciallyavailable Fibre Channel transmitter. At step 812, the transaction packetis received by receiver 710 and 10 b/ 8 b decoding is performed on thetransaction packet. Receiver 710 may be a commercially available FibreChannel receiver.

[0052] At step 814, character translation unit 712 may replace any ofthe unused control symbols which were inserted into the packet bycharacter translation unit 708 with the corresponding data symbols whichwere originally in the data stream. At step 816, it may be determined ifthe transaction packet includes any invalid symbols. This function maybe performed by character translation unit 712 or may be performed by aseparate logic device, such as a PLD or ASIC. If the transaction packetincludes no invalid symbols, then it may be passed to Reed-Solomondecoder 714. If the transaction packet includes an invalid symbol, thenit may be determined at step 818 if the invalid symbol is a controlsymbol (e.g., K28.5 symbol) or a data symbol. If it is a data symbol,then the packet may be passed to Reed-Solomon decoder 714. If theinvalid symbol is a control symbol, then the invalid symbol may simplybe replaced with the proper control symbol (step 820).

[0053] At step 822, Reed-Solomon decoder 714 performs error detectionand correction on the data stream. Similar to Reed-Solomon encoder 704and character translation unit 708, Reed-Solomon decoder 714 andcharacter translation unit 712 may be implemented as a single ASIC orPLD or separate ASICs or PLDs. Further, character translation unit 712and Reed-Solomon decoder 714 may be implemented as part of CDP 716 aremay be implemented as a separate unit. Finally, the data stream ispassed to receiving agent 824 (e.g. CDP 716).

[0054] Having thus described various embodiments of the presentinvention, additional alterations, modifications, and improvements willreadily occur to those skilled in the art. Accordingly, the foregoingdescription is by way of example only, and not intended to be limiting.The invention is limited only as defined by the following claims and theequivalents thereto.

What is claimed is:
 1. A method of transferring on a data link a packethaving a plurality of data symbols from a set of symbols and at leastone framing symbol from the set of symbols, the set of symbols includinga subset of valid symbols and a subset of invalid symbols, the methodcomprising: mapping at least some possible corruptions of the at leastone framing symbol, that result in valid symbols, from the subset ofvalid symbols to the subset of invalid symbols; and replacing, in thepacket, at least some of the valid symbols mapped in to the subset ofinvalid symbols, with unused symbols in the set of symbols.
 2. Themethod of claim 1, further comprising: adding error correctioninformation pertaining to the plurality of data symbols to the packet.3. The method of claim 2, further comprising: encoding the packet in aformat suitable for transmission.
 4. The method of claim 3, whereinencoding the packet in a format suitable for transmission furthercomprises performing 8 b/ 10 b encoding on the packet.
 5. The method ofclaim 3, wherein encoding the packet in a format suitable fortransmission further comprises performing 7 b/ 10 b encoding on thepacket.
 6. The method of claim 2, further comprising: transferring thepacket on the serial link.
 7. The method of claim 6, further comprising:indicating an error in the packet if the packet includes a symbol fromthe subset of invalid symbols.
 8. The method of claim 7, furthercomprising: determining if the error in the packet is in a data symbolor framing symbol based on the location of the error in the packet. 9.The method of claim 6, further comprising: recovering the valid symbolsmapped into the subset of invalid symbols by replacing at least some ofthe unused symbols in the packet with the valid symbols mapped into thesubset of invalid symbols.
 10. The method of claim 9, furthercomprising: applying an error correction algorithm to the packet tocorrect errors in the plurality of data symbols based on the errorcorrection information.
 11. A system for transferring on a data link apacket having a plurality of data symbols from a set of symbols and atleast one framing symbol from the set of symbols, the set of symbolsincluding a subset of valid symbols and a subset of invalid symbols, thesystem comprising: a mapper for mapping at least some possiblecorruptions of the at least one framing symbol that result in validsymbols, from the subset of valid symbols to the subset of invalidsymbols; and a first character translation unit for replacing, in thepacket, at least some of the valid symbols mapped in to the subset ofinvalid symbols, with unused symbols in the set of symbols.
 12. Thesystem of claim 11, further comprising: a first forward error correctionunit for adding error correction information pertaining to the pluralityof data symbols to the packet.
 13. The system of claim 12, furthercomprising: an encoder for encoding the packet in a format suitable fortransmission.
 14. The system of claim 13, wherein the encoder performs 8b/ 10 b encoding on the packet.
 15. The system of claim 13, wherein theencoder performs 7 b/ 10 b encoding on the packet.
 16. The system ofclaim 12, further comprising: a transmitter for sending the packet onthe serial link; and a receiver for receiving the packet from the seriallink.
 17. The system of claim 16, further comprising: an error detectionunit for indicating an error in the packet if the packet includes asymbol from the subset of invalid symbols.
 18. The system of claim 17,wherein the error detection unit further comprises an error resolutionunit for determining if the error in the packet is in a data symbol orframing symbol based on the location of the error in the packet.
 19. Thesystem of claim 16, further comprising: a second character translationunit for recovering the valid symbols mapped into the subset of invalidsymbols by replacing at least some of the unused symbols in the packetwith the valid symbols mapped into the subset of invalid symbols. 20.The system of claim 9, further comprising: a second forward errorcorrection unit for correcting errors in the plurality of data symbolsbased on the error correction information.